The present invention relates to electric watthour metering systems and, more particularly, to watthour metering systems including an electronic register containing a plurality of items to be displayed.
Electric watthour metering systems conventionally employ means for measuring instantaneous power consumption by a load and means for accumulating the instantaneous power consumption to provide a measurement of electric energy consumed by the load.
The most familiar type of power-measurement device includes a metallic disk driven at a speed proportional to the product of the line voltage and the load current, that is, the power consumed by the load. Each revolution of the metallic disk signifies the consumption of a predetermined quantum of electric energy. In some equipment, a pulse initiator converts the revolutions of the disk into electrical output pulses, each signifying the consumption of a predetermined quantum of electric energy.
More recently, fully electronic power-measurement devices are disclosed which directly measure consumption of electric power without the intermediate step of rotating a disk. These electronic devices produce an output pulse upon sensing the consumption of a predetermined quantum of electric energy.
In either type of power-measurement equipment, an electronic register conventionally sums the pulses to produce a total of energy consumption.
Utilities generally consider that the cost of electric energy can be attributed to two factors: 1) the out-of-pocket cost for generating the energy (fuel, operating and maintenance personnel and equipment, etc.) and 2) the capital cost of the generating equipment which must be installed to generate the electric energy.
Electric energy consumption is far from constant over a day, month or year. In some seasons such as, for example, the air-conditioning season, power demand is much greater than at other seasons. In order to provide adequate service, the utility must provide generating equipment capable of satisfying the maximum demand which it may experience. Thus, the utility must bear the capital cost of installing sufficient generating capacity to satisfy the peak demand. It must do this while knowing that, except at peak times, a substantial portion of its generating capacity remains idle. According to this analysis, the capital component of the total energy cost is governed by the peak demand in the peak season.
Some rate-setting bodies have recognized the importance of peak demand and have established tariffs in a manner designed to encourage energy consumers to limit peaks in their energy usage and to shift energy consumption from peak to off-peak times. One of such tariffs permits the utility to measure the amount of energy consumed in a sequence of demand intervals. At the end of each demand interval, the demand in the just-completed demand interval is compared with a stored value representing the highest demand in any demand interval since the register was last reset. If the demand in the just-completed demand interval is higher than the previous high demand value, then the value of the demand in the just-completed demand interval is stored and the previously-stored value is deleted. The value of the maximum demand existing at the time the meter is read determines the rate the consumer must pay for all of its electric energy. Thus, the consumer receives a powerful economic incentive to limit the peak load placed on the utility system.
An electronic register including demand metering and time-of-use metering require constantly updated time and date information for setting the lengths of the demand intervals and for switching between the permitted rates at different times and seasons.
The data in an electronic register is displayed on a human-readable display device for providing access to the data stored therein. Conventional display devices include, for example, light-emitting diodes and liquid-crystal displays. In order to limit cost, and to conserve space, such display devices are capable of displaying far less than the total amount of information contained in the electronic register. It is thus conventional to display the data sequentially, one or two lines at a time, holding each displayed line on the display for long enough for a person to see, and possibly to write down, the relevant information. Each item may remain displayed for several seconds. In some operations for electronic registers, a large number of separate items require sequential occupancy on the display wherein each occupancy requires several seconds to complete. The complete display sequence, sometimes called a scroll, may take up to several minutes.
The extended time for running a complete display scroll produces a problem in interpreting the data. In conventional systems, each displayed item represents the value of the displayed data at the time that it is displayed. In a scroll lasting for several minutes, the later-displayed data may not relate to the earlier-displayed data. For example, during the scroll, the electronic register may have passed from one demand interval to another, and may have recorded a new maximum demand, and have zeroed the data in the current-demand period. One examining the scrolled display may find it difficult to understand the relationship between present, very small, amount of energy accumulated in the current-demand period and the previously-displayed data of the timing of the demand interval itself which showed that the end of the demand interval was being approached. As a further example, during the scroll, the time of day may have passed from one time-of-use accumulation to another. Combining earlier- and later-displayed data of this type may be misleading.